a. Field of the Invention
The present invention relates to an electric power conversion system using semiconductors, which is used in the field of power system, electric railway, large-scaled plant or the like, particularly for AC-to-DC conversion, DC power transmission or frequency conversion system in those fields. It is particularly suitable for use in a variable-speed electric generator-and-motor which is required to continue its running to the limit even in case where AC input voltage is unstable owing to abnormal conditions in the electric power system.
b. Background Art
A line commutation type semiconductor electric power conversion system is used in the hope of improving the stability of post-conversion power supply. As a problem, however, the thermal capacity of power semiconductors such as thyristors is small, compared with the thermal capacity of transformers, electric machines and apparatuses, and therefore, it is most likely that a protection device for such electric power conversion system is designed to be very sensitive to abnormal conditions. As a result, even in case of appearance of minor disturbance on the power supply side, undesired interruption will be caused in the system using such semiconductor electric power converter, and therefore the stability of post-conversion power supply will be lowered.
In an attempt to solve such problem Japanese Patent Application Laid-Open (KOKAI) No. 63-52699 proposed a semiconductor electric power conversion system as shown in FIG. 14.
FIG. 14 shows a conventional electric power conversion system as comprising transformer 2 connected to power system line 1, three-phase bridge circuit 3 connected to transformer 2, current transformers 5 to detect three-phase input currents; current transformers 6 to detect DC output current; input current detection circuit 7 connected to current transformers 5; absolute value arithmetic operation circuit 8 connected to a joint between input current detection circuit 7 and current transformer 6; differential current detector 9 connected to absolute value arithmetic operation circuit 8; overcurrent detector 10 connected to input current detection circuit 7; and means 11 connected both to differential current detector 9 and overcurrent detection 10 for making a decision as to whether running be continued or not.
The AC input current Iac from input current detection circuit 7 will be equal to the DC output current Idc from DC current transformer 6 during a normal operation. A signal representing the difference .DELTA.I between these currents Iac and Idc is rectified in absolute value arithmetic operation circuit 8, and the rectified signal .DELTA.I is directed to differential current detector 9. On the other hand, the signal representing the AC input current Iac is directed to overcurrent detector 10. An apparatus 11 for making a decision as to whether or not the power conversion system is allowed to continue its running, has the functions as shown in FIG. 16.
In FIG. 16 if the level of the signal OC appearing at the output terminal of overcurrent detector 10 is "0" (the level of the signal being "1" upon detection of overcurrent) (Step 100), it is decided that the DC output current from three-phase bridge 3 is of normal value, and that nothing is wrong (Step 104), and then operation continuing instructions GO1 will be dispatched (step 107), allowing the power conversion system to continue its running.
In case where a return is made from suppression control to normal operation even if the signal OC appearing at the output terminal of overcurrent detector 10 is "0", return-to-normal operation processings will be required (Step 109). First, a decision will be made as to whether overcurrent suppression control instructions G02 were dispatched before (Step 108). In the affirmative the return-to-normal operation processings will be performed, and then operation continuing instructions GO1 will be dispatched (Step 107).
The contents of the return-to-normal operation processings (Step 109) are as follows
i) While the situation remains as it was prior to detection of overcurrent, a part of the arithmetic operation of control means (not shown) for controlling the gate electrodes of the thyristors in the three-phase bridge circuit, is initialized on the basis of the circuit currents at present.
ii) Initialization is made by teaching the control means the current firing condition of forward or backward thyristors TYS.
On the other hand, if the signal OC appearing at the output terminal of overcurrent detector 10 is "1", a decision will be made as to whether .DELTA.I is within an allowance or not (Step 101). If .DELTA.I is within allowance K1, and if nothing is wrong with the generator, it is decided that the cause for the overcurrent is a power system fault or malfunction of switching devices in other power conversion systems (Step 110 and 103), and then overcurrent suppression control instructions G02 will be dispatched (Step 106).
In response to the overcurrent suppression control instructions G02, shortcircuiting switch TYS4 is fired, thereby suppressing the current, which otherwise, would be increased, in the three-phase bridge circuit. Also, the signal representing the overcurrent suppression control instructions G02 will be stored, for instance by using a flip-flop circuit (not shown) to prepare for the return-to-normal operation proceedings (Step 109).
In case where the difference .DELTA.I represented by the signal appearing at the output terminal of absolute value arithmetic operation circuit 8, is above the value of allowance K1, it will be decided that it is an overcurrent which was caused by inner faults or defects of the three-phase bridge circuit 3 (Step 102), and then emergent interruption command ST will be dispatched (Step 105).
In response to the emergent interruption command ST the firing signals will be prevented from being directed to the thyristors TY1 to TY6 in the three-phase bridge circuit 3.
It is possible that the difference .DELTA.I remains within allowance K1 even in case of inner defects in other power conversion systems, and therefore, a decision will be made as to whether the defect is internal or external one after the decision of Step 101 (Step 110). If it should be found to be an inner defect (Step 111), no overcurrent suppression control will be carried out. Instead, as is the case with the inner defect of the power conversion system, emergent interruption instructions will be dispatched (Step 105).
The above described prior art semiconductor electric power conversion system is not satisfactory in detection of abnormal conditions because of no capability of making a distinction between abnormal conditions caused by defects in the three-phase bridge circuit and associated parts, and abnormal conditions caused by commutation failures in the three-phase bridge circuit, which commutation failures may be caused by disturbances on the AC power supply side or on the DC power output side. This results in delay in detection of inner defective conditions, and emergent interruption of power conversion is liable to take place even at the time of appearance of abnormal conditions caused by external disturbances. Accordingly, the reliance of operation is lowered.